Astria surges on BioCryst's $700 mln buyout deal

** Shares of drug developer Astria Therapeutics ATXS.O up 43% at $12.12, its biggest one-day percent gain since 2021

** BioCryst Pharmaceuticals BCRX.O says it will buy Astria for $700 million in cash-and-stock deal

** The offer implies a per-share value of $13, a premium of about 54% over Astria's last close

** The deal adds experimental injectable drug navenibart, which is in late-stage testing for hereditary angioedema (HAE), to BioCryst's portfolio

** HAE is a rare genetic condition that causes repeated episodes of severe swelling in areas like the limbs, face, gut and airway, says U.S. FDA

** Shares of BioCryst fall nearly 5% to $6.83

** Up to last close, BCRX stock down ~6%, ATXS stock down ~5% YTD